Method and column for collection and separation of oil, gas and water from blowing wells at the sea bed

ABSTRACT

Method by collection and separation of oil, gas and water from an offshore oil/gas well (2) and a column (1) for usage by the same. 
     The column (1) comprising a vertically arranged tube (3) with a lower end (8) resting on the sea bed (14) and an upper closed end (9) from which gas may be discharged by gas outlet means (6). Oil-gas mixture flowing out of a well head (22) in operation of the column (1) will be retarded by an oil column (10) in the tube (3), thereby releasing gas which is collected in the upper portion (11) of the column. Motion of the oil at the surface of the oil column (10) will be very small, oil thereby flowing over an overflow rim (15) into an overflow channel (26), from where oil is transferred to the sea surface by oil outlet means (7). 
     The motion of the mixture may be additionally dampened by horizontal webs. 
     The column (1) may be operated at sea depths more than 300 meters and at shallow water where the column may be constructed as part of a platform.

This invention relates to a method by collection and separation of oil,gas and water from an oil/gas well and a column for usage by the same.

During the last years many attempts have been made to control oil andgas streaming out of blowing oil/gas wells on the sea bed. Such attemptsare undertaken to avoid pollution of the surrounding sea and sea-shores,highly being a danger for damaging marine life and pollutioning largesea-shore areas. Additionally high waste economic losses follow flowingof such wells.

Existing equipment of to-day, such as booms, skimmers, sombreros etc.have proved to be insufficient under prevailing weather conditions.Therefore, new ways had to be developed to collect and separate gas andoil blowing uncontrolled from wells at the sea bed.

Means therefore are required, which can collect and commercially utilizegas and oil from such wells during the period of time in which othermeans are working to control the blow out, e.g. drilling relief wells.

Means in the shape of sombreros, e.g. according to U.S. Pat. No.3,664,136, have been put into action to avoid pollution of the sea waterand the surroundings. The intention of such sombreros has been tocollect the oil-gas mixture blowing out of the subsea oil well.Principal reasons for failure of such sombreros have been escape of oiland gas below the edge of the sombrero and the attempt to transfer theoil-gas mixture, usually from the top of the sombrero to the seasurface. Caused by the proportional expansion of the gas escaping fromthe oil-gas mixture, such attempt has serious problems during thesignificant expansion of the gas volume by transportation of the oil-gasmixture in risers up to the sea surface.

One criterion by controlling a blowing well using a structure arrangedon the well is to separate oil from gas and to control the twocomponents individually. Furthermore, the pressure increasing within thestructure caused by the flowing gas-oil mixture has to be controlled andlimited to a pressure which does not exceed the strength of the bearingsoil. Otherwise damage to soil layer and oil/water piping will occur.

Separation of the gas from the oil, as gas and oil have quite differentbehaviours as to pressure drops and expansion, large pressurefluctuations caused by gas bubbles in transportation risers from the seabed to the sea surface as well as cavitation problems in the pumps, willbe required to avoid such problems.

For the requirements it should be demanded of structures as follows:Insignificant or no soil failure, limitation of pressure fluctuations,insignificant or no damage to flowlines on or near the sea bed,operational independency of water depths, easy and ready installation,reliability and mobility. Furthermore, demands to economicalconstruction and maintainance as always will be important.

The problem of soil failure will arise when the structure covering thewell head and having an open lower end placed on the sea bed, is filledwith the oil-gas mixture from the well, causing pressure differencesinside/outside the structure. If the pressure difference outside andinside the structure near the sea bed exceeds 3-5 m H₂ O, a breakdown ofthe soil normally will be expected. Such breakdowns normally will causeleakage at the sea bed surface or in the soil. Pressure fluctuations atthe sea bed, e.g. at 300 m water depth, vary a lot more than the soillimit of 5 m H₂ O. This low pressure difference limit of approximately 5m H₂ O makes it necessary to be able to minimize the fluctuations inoil/gas pressure within any structure placed open to the sea bed.

While the pressure at the bottom of a structure will be equal to theheight of the liquid in the structure times the specific gravity of theliquid plus the atmospheric pressure, the gas pressure within such astructure will be the same in all directions.

If gas and oil are transferred together through a riser where thepressure in the riser sections is depending upon the vertical positionof the sections, the bottom pressure continuously will vary from time totime with the gas content in the riser as the hydrostatic pressure atthe bottom causes the gas to displace the oil while expanding during itstravel up the riser.

By separating the gas from the oil in such a degree that the gas contentin the oil is drastically decreased, the fluctuations in the riserbottom pressure also will be substantially decreased. Such a separationis possible by obtaining a free oil level in a column, establishing anoverlaying hat for the gas released from the oil. Such a gas hat therebyforming the top of a structure arranged over the blowing well.

The method and structure of the present invention avoid the failures anddisadvantages of prior structures for collecting and/or separating oiland gas from blowing wells. According to the present invention themethod comprises lowering of a vertically arranged column over a blowingwell by supplying ballast to ballast tanks connected with the column,until the column rests on the sea bed around the well head, guidingoil-gas mixture from the well head into the column, thereby building upan oil column in the interior of the structural column and an upper gasportion in the column, the upwards directed motion of the oil-gasmixture thereby being retarded such that the motion of the mixture atthe oil surface being substantially insignificant, the pressure and theamount of oil and gas in the structural column being controlled byactuation of valves for oil and gas outputs, the hydrostatic pressureoutside and inside the lower edge of the column resting on the sea bedbeing substantially equal, thereby maintaining a gas portion in theupper portion of the column by discharging gas from the column throughthe sea surface and discharging oil from the oil column also to the seasurface.

A structure according to the present invention may have the shape of acolumn comprising a vertically arranged tube with an upper closed endhaving outlet means for gas, a lower end of the tube having means forsupporting the column at the sea bed and the middle portion of thecolumn having oil outlet means.

An overflow rim may be provided below the upper portion of the tube,whereby remaining gas in the upper part of the oil column is releasedfrom the oil well when the oil is flowing over the rim, the outlet meansfor oil being provided in an overflow channel below the overflow rim andbetween the rim and the interior wall of the tube.

The interior of the column may be provided with one or moresubstantially horizontal webs acting as dampers to the upwardly directedmotion of the oil-gas mixture.

An outer casing may be provided around the column forming therebetweenballasting and storing tanks as well as installing spaces for valves,pipelines and tanks provided to control lowering and flotation of thecolumn and the pressure inside the column when the column is inoperation. A deck may be arranged, resting on the top of the casing,suitable for supporting necessary equipment for the operation of thecolumn, the equipment on the platform being accessible for divers whenthe column is in operation at great depths.

From the column, oil and gas are separately transferred to the seasurface where further treatment may take place on barges, ships,platforms etc. before further transportation, or the gas may be burned.

The risers for such transfer of oil and gas may be fixed or flexible,depending on water depths and other circumstances. It is, however,anticipated that flexible risers might be the most economical solutionfor great depths, as such risers also allow the use of the system atdifferent water depths without costly rearrangements.

The column principle can be applied for all water depths, subject toestablishing a stage of pressure equilibrium, which is set by thefollowing equation:

    P+ρ.sub.1 Hg=P.sub.1 +ρ.sub.2 h.sub.2 g

where

P=atmospheric pressure at sea level,

ρ₁ =specific gravity of water

H=water depth,

P₁ =gas pressure in top of column,

ρ₂ =specific gravity of oil in column,

h₂ =height of oil in column

In order to limit the possibility of oil leakage under the bottom wallof the column, due to small pressure fluctuations, the bottom pressurein the column should be kept within the following ranges:

    P+ρ.sub.1 Hg>P.sub.1 +ρ.sub.2 h.sub.2 h>P+ρ.sub.1 Hg-P.sub.s

giving a pressure control span of P_(s) where P_(s) =maximum soildifferential pressure before soil breakdown.

If leakage under the bottom is allowed or controlled by means of skirtspenetrated into the sea bed, where flow lines do not prevent this, thebottom pressure in the column can vary as follows:

    P+ρ.sub.1 Hg+P.sub.s >P.sub.1 +ρ.sub.2 h.sub.2 >P+ρ.sub.1 Hg-P.sub.s

giving a control span of 2P_(s).

During operation the oil from the well will flow freely into the column,the gas will separate and the system will establish its own state ofequilibrium. Pressure built up at the bottom is avoided by throttling abottom valve and/or a top valve. The oil in the column is flowing overthe overflow rim prior to entering the transfer risers which willimprove the separation of gas.

The dynamic energy in the blowing oil-gas mixture flowing upwards isdampened by the existing oil and water liquids in the column. To achievethis, the amount, diameter ahd height of water and oil in the columnmust be large enough to dampen and absorb the dynamic energy from theblowing oil-gas mixture. Furthermore, the dimensions must be largeenough to let gas bubbles rise and expand without creating largefluctuations in the hydrostatic pressure at the bottom. This effect willdecrease with increasing diameter and height. The required height of theoil/water column might be reduced by installation of mechanical damperor dampers in the column.

The invention is fully described by way of example with reference to theaccompanying drawings, in which:

FIG. 1 shows an elevation of the column according to the presentinvention,

FIG. 2 shows the column of FIG. 1 with an outer casing, ballasting andstoring tanks and an equipment deck accessible for divers as the columnmay be in operation on a sea bed,

FIG. 3 shows a column constructed for shallow water with a platform deckabove the sea surface, and

FIGS. 4 and 5 show cross-sections according to FIG. 2.

A vertically arranged column 1, in operation covering an oil/gas well 2,comprises a tube 3 having valves 4 and 4a and valve 5 constituting partsof outlet means 6 and 7 for gas and oil, respectively. The lower end 8of the column 1 has a lower edge 12 resting on the sea bed 14. In thevertical tube 3 and the lower end 8 of the column 1 an oil column 10will be built up during operation. An upper end 9 of the column containsan upper gas-filled portion 11 below which an overflow rim 15 isarranged for releasing gas from the oil-gas mixture as the mixture flowsover the rim 15 and into an overflow channel 26 from which the oil istransferred to the sea surface by the outlet means 7. The lower end 8 ofthe column 1 is provided with a valve 18 for discharging water from thelower part of the column 1 and for buoyancy purposes by floating andlowering the column.

Oil-gas mixture flowing out of the well 2 through the well head 22 willbe retarded by the fluid of the oil column 10. Gas will be released fromthe oil-gas mixture and find its way to the gas-filled portion 11. Atthe surface of the oil column 10 the motion of the oil-gas mixture isretarded to such an extent that the oil is substantially calm and mostof the gas is released from the mixture. Possible remaining gas will bereleased from the mixture when flowing over the overflow rim 15 into theoverflow channel 26. Water separated from the oil-gas mixture willestablish a water-filled lower portion in the lower end 8 of the column1, which water can be discharged from the column by valve 18. Valve 18also may be used as supplementary discharging possibility to the outletmeans 7 for oil by excessive flow into the column of oil-gas mixture.

Surrounding the column 1 an outer casing 13 is arranged spaced from thetube 3, thereby providing facilities for e.g. piping systems.Surrounding the casing 13 ballasting and storing tanks 27 may bearranged around the lower portion of the casing 13.

Water may be discharged from the lower end 8 of the column 1 through amanifold 17 to the surrounding sea or by valves 18 and 19 to storagetanks 27, from where it may be discharged by a pump 29 to thesurrounding sea. The storage tanks 27 hereby being in operation asseparating tanks for oil and water, the oil separated in the upper partof the tanks 27 may be discharged through a manifold 16 from which oilmay be transferred to the sea surface or to a manifold 20 through whicha pump 30 may discharge oil from the overflow channel 26 to the seasurface for further treatment.

To increase the retarding function of the oil column 10, one or morehorizontal webs 21 may be arranged in the interior of the tube 3, suchwebs being designed as stiffeners for the tube 3.

The lower edge of the tube 3 may be designed as webs, boxes or sectionsof which certain ones may be removed to avoid damage on objects on ornear the sea bed 14 around the well head 22, such as pipelines.Furthermore, the lower edge 12 may be designed to penetrate the sea bedthereby allowing a certain pressure difference between the inside andthe outside of the lower end 8 of the column 1.

To avoid damage on flowlines and bottom risers at the sea bed, the loweredge 12 of the column 1 partly or totally may be provided with awater-filled flexible rubber cushion distributing the load of the columnevenly on the sea bed.

The column 1 may be provided with an equipment deck 31 supportingnecessary equipment for the piping systems of the entire column 1 andbeing accessible for divers when the column is in operation.

Further equipment is installed for purposes of measuring pressures atdesired points of the column, the casing, the tanks, pumps, manifoldsand the gas and oil transmitting devices 23, 24 respectively, and thedischarging devices 6, 7 for gas and oil, respectively.

In operation the pressure in the lower end 8 of the column 1 by means ofthe afore-mentioned equipment and controlling devices, is maintainedsubstantially equal to the pressure outside the column at the sea bed.

The column design is strongly related to the physical conditions underwhich the column will be in operation and towed offshore to a blowingwell. Fulfilment of such requirements consequently will exceed therequirements put forward from the pressure conditions connected with theoperation of the column, the column thereby being operational at depthsdown to more than 300 meters as well as in shallow water.

The column 1 may, if beneficial, also be used as an extra safety marginduring drilling by placing the column over the bottom installed B.O.P.and drilling through the top of the column which has to be designed andequipped for such a purpose.

For use in shallow water the column may be provided with a fire wall andfire fighting devices withstanding burning oil and gas at the seasurface when installing the column.

Ballasting and storing tanks 27 may be used for trimming the load on thecolumn structure when standing at the sea bed in order to keep it stabledependent on soil conditions of the particular well, in addition toballasting during submergence and storing purposes for oil and/or waterduring operation.

During submergence valves 4, 4a, 5 and 18 are open to allow free flow ofgas and oil through the column. After installation at the sea bed andrequired ballasting, the valves carefully are set and the oil pumps putinto operation, the valves, pumps and other equipment being remotelycontrolled through lifelines from an operation barge, such equipment andpiping systems normally being automatically controlled.

I claim:
 1. A balancing column for collecting and separating gas, waterand oil flowing from an underwater well head, said column comprising:(a)a vertically arranged column, said column having ballasting means forlowering said column over said well head in a vertical position, saidcolumn having a closed upper end and an open lower end for supportingsaid column on the sea bed; (b) an oil accumulator in the lower andmiddle portion of the column for dissipating the kinetic energy of anupflowing oil-gas mixture; (c) a gas accumulator in the upper end of thecolumn for accumulating gas as it separates from said oil-gas mixture;(d) first and second valve means for equalizing the hydrostatic pressureinside and outside the lower edge of the column resting on the sea bed,said valve means regulating the accumulated oil and gas within thecolumn to provide a positive engagement between the column and the seabed, said first valve means discharging oil from said oil accumulatorand said second valve means discharging gas from said gas accumulator atpredetermined rates for said equalization; whereby the positiveengagement between the column and the sea bed is maintained to prevent abreakout of the sea bed below the column.
 2. A balancing column forcollecting and separating gas, water and oil flowing from an underwaterwell head as claimed in claim 1 which further comprises an oil-gasseparator means mounted between said oil accumulator and said gasaccumulator.
 3. A balancing column for collecting and separating gas,water and oil flowing from an underwater well head as claimed in claim 2which further comprises an overflow rim mounted below said gasaccumulator adjacent said first valve means, whereby any remaining gasin the oil will be released when oil flows over the said rim.
 4. Acolumn according to claim 1 or 2 wherein said column is provided with atleast one horizontal interior web for damping the upwardly directedmotion of the oil-gas mixture.
 5. A column according to claims 1 or 2wherein the second valve means further comprises at least one conduitfor connection with storage means for gas on the sea surface.
 6. Acolumn according to claims 1 or 2 wherein said second valve meansfurther comprises a valve vertically arranged in the top of the column,allowing drilling operations from the sea surface through the valve andthe entire height of the column.
 7. A column according to claims 1 or 2wherein said first valve means further comprises a manifold and at leastone conduit for connection to oil storage means on the sea surface.
 8. Acolumn according to claims 1 or 2 which further includes a tubularcasing arranged around the column, thereby providing a space between thetube and the casing to install valves, pumps and pipes, said casingprotruding upwardly from the lower end of the column.
 9. A columnaccording to claims 1 or 2 that further includes outwardly extendingsectional means for supporting the column on the sea bed, said meanshaving sections which may be removed to avoid damage to objects on ornear the sea bed around the well head.
 10. A column according to claims1 or 2 wherein the lower end of the column is provided with a thirdvalve means for transfer of liquid into and out of the lower end of thetube.
 11. A method of collecting and separating gas, oil, and waterflowing from an under water well head, said method comprising:(a)ballasting a vertically arranged column over the well by filling one ormore ballast tanks mounted on the column, until the column rests on thesea bed above the well head; (b) filling the interior of the column withthe oil-gas mixture from the well head to form a vertical column of oil;(c) retarding the kinetic energy of the oil-gas mixture by directing theup-flowing mixture into said column of oil; (d) separating the oil andgas mixture and providing a gas accumulator in the upper portion of thecolumn; (e) equalizing the hydrostatic pressure inside and outside thelower edge of the column resting on the sea bed by regulating theaccumulated gas pressure within the gas accumulator; whereby a positiveengagement between the column and the sea bed is maintained to prevent abreakout of the sea bed below the column.
 12. A method of collecting andseparating gas, oil, and water as claimed in claim 11 which furthercomprises the step of regulating the flow of oil from the column of oilto equalize the hydrostatic pressure between the inside and outside ofthe lower edge of the column.
 13. A method of collecting and separatinggas, oil, and water as claimed in claim 11 or 12 wherein the hydrostaticpressure is equalized by adjusting the oil and gas pressures as follows:

    P+p.sub.1 Hg=P.sub.1 +p.sub.2 h.sub.2 g

wherein: P=atmospheric pressure at sea level, p₁ =specific gravity ofthe water in which the column is located H=the water depth at the seabed p₁ =the gas pressure in the top of the column p₂ =the specificgravity of oil in the column, and h₂ =the height of the oil in thecolumn.
 14. A method of collecting and separating gas, oil, and water asclaimed in claim 11, wherein the oil pressure at the bottom of thecolumn is maintained within the following ranges:

    P+p.sub.1 Hg>P.sub.1 +p.sub.2 h.sub.2 h>P+p.sub.1 Hg-P.sub.s

wherein P_(s) is the maximum pressure permissible before soil breakoutoccurs.
 15. A balancing column for collection and separation of gas,water and oil which, controlled or not, is flowing out of an oil/gaswell on a sea bed, characterized in that the column comprises avertically arranged tube with an upper closed end having an outlet meansfor gas, a lower end of the tube having means for supporting the columnon the sea bed, a middle portion of the tube having an oil outlet meansfor tranferring oil to the sea surface, and an overflow rim mounted insaid tube below an upper portion of the tube that collects release gas,whereby remaining gas in the oil will be released when oil flows overthe rim, said oil outlet means opening into an overflow channel betweenthe interior wall of the tube and the rim at a level below the rim, anda lower end of the tube having valve means for liquid passage throughthe lower end of the column to equalize the hydrostatic pressure insideand outside the lower edge of the column when in operation.